A solar cell supplies power by using electrons that are excited by solar light irradiation on a semiconductor material. In order to improve efficiency of the solar cell, it is useful to know energy of the electrons excited by the solar light. The solar light includes the long-wavelength light that cannot excite the electrons in the semiconductor material to the vacuum level. Other than the solar cell, there are cases where it is useful to measure energy of the electrons excited by irradiating the long-wavelength light that cannot excite the electrons to the vacuum level.
To measure energy of the electrons excited within the sample, it is effective to disperse or split photoelectrons emitted outside the sample from a surface of the sample according to kinetic energy of the photoelectrons. The measurement needs the photoelectrons emitted outside the sample.
Since energy of the electrons excited by the visible light or the infrared light do not reach the vacuum level of the sample in most cases, the photoelectrons are not emitted outside the sample. It makes it difficult to measure energy of the electrons excited by irradiating the visible light or the infrared light.
Thus, a technique of Document I was developed therefor. The technique of Document 1 realizes a state in which a vacuum level of a sample is lower than the lowest energy level of a conduction band (NEA (Negative Electron Affinity)) by processing a surface of the sample. This technique enables measurement of energy of the electrons, which are excited by the visible light or the infrared light, and which would not be emitted outside the sample unless the aforementioned processing is carried out. Electrons having energy not exceeding the vacuum level before the processing are not emitted outside the sample unless the vacuum level of the surface of the sample is lowered.